Consumers continue to demand complex functional circuits, with higher performance and lower cost, such as data processors, very large scale integrated circuits (VLSI), systems on chips (SoCs), central processing units (CPUs), advanced processing units (APUs), graphics processing units (GPUs), memory sub-systems, system controllers, and peripheral functions. At the same time, testing the operation of the functional circuit and determining that the functional circuit is operating as desired is increasingly difficult. Robust testing technologies have been developed such as boundary scan, now defined as a standard by the Joint Test Action Group (JTAG). Also, Automatic Test Pattern Generation (ATPG), which is compatible with many automatic test equipment systems, is used to determine if a functional circuit is operating according to specification. More advanced “at speed” ATPG technologies launch test patterns and capture the resulting data in a manner that tests a functional circuit at substantially the same speed the functional circuit runs during normal operation.
However a problem arises when testing circuits at speed by using a test pattern received and launched from an internal scan chain. The scan operation involves shifting bits of data serially through the scan chain at a relatively low speed. Once the test is enabled, however, a relatively large number of logic gates switch at high speed and their outputs are captured in the scan chain. This scan-to-test transition can cause a significant disruption on the internal power supply voltage, and the disruption can distort the test and cause circuits that operate according to specifications to fail.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.